Multistage thermostats have been used for a long time to control stages of heating wherein a refrigeration heat pump is used for one stage and auxiliary electrical strip heat is used for other stages. Also there has been a continual need for protection for refrigeration apparatus when exposed to cold temperatures such as electrical heaters for the outdoor compressor unit for maintaining the unit warm during the inoperative period. When heat pump apparatus is used for heating as the first stage in a multistage control system, during normal temperature control at a high heating load with low outdoor temperatures, the compressor is almost continually operated.
When a multistage thermostat control system is used with a night setback thermostat, the controlled temperature desired in the space is setback to some lower level during a selected period of time. During a portion of the setback time, there may be no call for heat and thus the refrigeration heat pump can remain idle for an extended period of time before a call for heat brings it into operation. During low outdoor temperatures, the starting of the heat pump after an extended inoperative period can result in damage. Additionally, if the space temperature is allowed to drop for a night setback during a heavy heating load with a low outdoor temperature, the pickup load to bring the temperature back to normal is extremely great. The control system may by necessity need to bring on the more expensive electrical strip heat for a pickup operation.
The present invention provides for a means responsive to outdoor temperature to maintain the refrigeration heat pump in operation when the outdoor temperature drops below some predetermined value during the setback time period. During light loads even though there is a low outdoor temperature, the continuously energized heat pump may result in an increased space temperature; therefore, the invention contemplates the use of a temperature responsive element mounted in the space to override the continuous energization of the heat pump should the space temperature exceed some predetermined value during the setback period.